Would it make sense to consider giving nsTHashtable and PLDHashTable
different names? Right now the names suggest that we have 3 general purpose
hash tables, but it might be easier if the names better suggested their
purpose, e.g. PLDHashTable -> MinimalCodeSizeHashTable (I'm sure we can do
better than that).
Similarly, would it make sense to consolidate the APIs, as much as
possible, if the primary differences are around implementation details like
"how much is templated/inlined"?
Alex
On Wed, Aug 15, 2018 at 5:39 AM Nicholas Nethercote <n.netherc...@gmail.com>
wrote:
> Hi,
>
> I recently moved Spidermonkey's js::Hash{Map,Set} classes from
> js/public/HashTable.h into mfbt/HashTable.h and renamed them as
> mozilla::Hash{Map,Set}. They can now be used throughout Gecko.
>
> (js/public/HashTable.h still exists in order to provide renamings of
> mozilla::Hash{Map,Set} as js::Hash{Map,Set}.)
>
> Why might you use mozilla::Hash{Map,Set} instead of PLDHashTable (or
> nsTHashtable and other descendants)?
>
> - Better API: these types provide proper HashMap and HashSet instances, and
> (in my opinion) are easier to use.
>
> - Speed: the microbenchmark in xpcom/rust/gtest/bench-collections/Bench.cpp
> shows that mozilla::HashSet is 2x or more faster than PLDHashTable. E.g.
> compare "MozHash" against "PLDHash" in this graph:
>
>
> https://treeherder.mozilla.org/perf.html#/graphs?timerange=604800&series=mozilla-central,1730159,1,6&series=mozilla-central,1730162,1,6&series=mozilla-central,1730164,1,6&series=mozilla-central,1732092,1,6&series=mozilla-central,1730163,1,6&series=mozilla-central,1730160,1,6
>
> Bug 1477627 converted a hot hash table from PLDHashTable to
> mozilla::HashSet and appears to have sped up cycle collection in some cases
> by 7%. If you know of another PLDHashTable that is hot, it might be worth
> converting it to mozilla::HashTable.
>
> Both mozilla::Hash{Map,Set} and PLDHashTable use the same double-hashing
> algorithm; the speed difference is due to mozilla::HashSet's extensive use
> of templating and inlining. The downside of this is that mozilla::HashSet
> will increase binary size more than PLDHashTable.
>
> There are overview comments at the top of mfbt/HashTable.h, and the classes
> themselves have more detailed comments about every method.
>
> Nick
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>
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